The electrode construction of high intensity discharge lamps is governed by multiple requirements that have to be fulfilled simultaneously for proper electrode operation. The lamps have to start reliably, and function properly under steady-state conditions. Starting and steady state operating regimes of the electrodes set different and often contradicting constraints for a suitable electrode structure.
During the starting (i.e. ignition) and the run-up transition phases of lamp operation, the electrodes run through the glow and the glow-to-arc transition modes with currents differing in orders of magnitudes. For a long useful product life, these transition phases have to be as short as possible in order to reduce electrode degradation due to sputtering by heavy particle bombardment from the discharge plasma and due to excess evaporation rate of electrode material close to or sometimes even above its melting-point temperature. In the course of these transition phases of electrode operation, discharge plasma is generated in the lamp and adequate energy transfer from the plasma to the electrodes is required in general. The transferred energy heats the electrodes up to temperatures where thermionic electrode emission assisted by electric field provides the required take-over current of the lamp to keep it in operation, and then brings it into steady-state conditions.
Once the electrodes have been heated up to their steady-state operating temperature, spatial temperature distribution of the electrodes have to be properly adjusted to provide the required discharge current at their interface area with the discharge plasma. On the other hand, appropriate temperature gradients not only across the electrode front face but also along the electrode axis have to be established to avoid excess evaporation of electrode material, flickering, arc anchor point movements, and overheating the electrode foot-points.
The set of requirements concerning the electrodes of high intensity discharge lamps with high take-over, run-up and/or steady-state operating currents, and especially of high intensity discharge lamps for automotive applications is even more demanding. In the case of high intensity discharge lamps for automotive applications, there are additional constrains set for the electrode shaft diameter, the electrode tip geometry and positioning, which are related to the performance of the lamp in optical projection systems (automotive headlamps). In addition, the requirements of ‘instant light’ generation and ‘hot re-start’ ability imply heavy lamp currents and heavy electrode overload during the starting and run-up transition phases of lamp operation. Automotive headlamps are generally heated with a power of 70 W to 90 W during lamp run-up, which power is gradually decreased to 35 W within approximately 30 s to reach rated steady-state lamp power value and lamp operation conditions. Consequently during this run-up phase, a substantial part of the electrode bodies is running at much higher temperatures compared to the steady-state conditions. This results in extremely high electrode foot-point temperatures, while the surrounding discharge vessel wall temperature is low: close to the temperature values of a non-operational lamp. The high spatial and temporal temperature gradients in the vessel wall at the hot electrode foot-points and beyond this point, that is in the sealing sections responsible for vacuum-tight closing of the discharge vessel (pinch seal sections), lead to extremely high thermally induced mechanical stress levels in the glass of the seal surrounding the electrodes. These thermally induced high mechanical stresses generate cracks and crack propagation in these pinch or shrink seal sections when the lamps are repetitively started and then switched off. This results in formation of leaking channels and in turn the loss of filling gas and dosing constituents of the discharge chamber, thus finally making the lamp inoperative. Such short-lived lamps severely affect product life performance and reliability, thereby road safety is also affected in a negative way, and vehicle maintenance costs are increased.
It is known from the prior art that the electrodes of high intensity discharge lamps often have a coil structure close to the electrode tip. The role of such coil component is partly to help ignition and partly to set the proper axial temperature gradients along the axis of the electrode, and especially in the area close to the electrode tip, via enhanced radiative cooling.
A metal halide lamp with such coil arrangement is disclosed e.g. in U.S. Pat. No. 4,105,908. The glow-to-arc transition of this known lamp is speeded up by using electrodes comprising an open tungsten wire coil on a tungsten shaft, the coil comprising two layers of a composite wire made by open-winding an overwind on a core and then close-winding two layers of the composite wire on the shaft. Although this structure decreases sputtering at starting and reduces glow-to-arc transition time, the disclosed coil structure is placed relatively close to the electrode tip, which is in contradiction with applicable standards set for high intensity discharge lamps by the automotive industry. Thereby, this known lamp cannot be used in this technical field.
A high-pressure electric discharge lamp is disclosed in U.S. Pat. No. 4,232,243. The electrodes thereof preferably comprise tungsten wire coils arranged relatively close to the electrode tip, which arrangement has the same disadvantages as above.
A HID lamp is disclosed further in U.S. Pat. No. 4,893,057. This known HID lamp incorporates ‘all-metal’ electrodes providing rapid transition of the arc to the electrode tip. The electrode comprises a length of thoriated tungsten wire having a close wrapped coil at tip ends, so that rapid heating of the electrode tip promotes rapid transition of the arc from coil crevices to the tip. Again, the coil is relatively close to the electrode tip and contributes exclusively to the ignition, instead of also limiting temperature at electrode foot-points.
The electrodes currently used in high intensity discharge lamps for automotive applications have a more simple geometry. These electrodes do not have a coil component on the electrode shaft at least definitely not inside the arc chamber. This is because these lamps have to be in conformance with some additional constraints, which is basically related to the optical design of the headlamps/projecting reflectors where these lamps are used. The strict constraints related to such optical considerations and the extremely compact geometry of the discharge vessel of these amps generally do not allow additional components to be arranged at and close to the tips on the electrode shaft. The axial temperature distribution of the electrodes is governed by a power balance between the input power at the electrode tip interfacing with the discharge plasma, the radiative and conductive/convective cooling on the cylindrical side surface of the electrode shaft, and the conductive power loss across the shaft cross section towards the electrode foot-point area.
It is also generally known in the art that a coil may be used on electrodes of high intensity discharge lamps of high operating currents to lower the thermal load on the glass wall at the electrode foot-point. In contrast with the coil located close to the tip of the electrode shaft described previously, such a coil is located outside the discharge chamber and surrounded by the wall material of the discharge chamber, i.e. it is ‘pinched’ into the bulk glass material of the glass-to-metal seal at the discharge chamber end section. Despite the advantage of this coil structure in increasing the electrode foot-point surface and thus lowering the power load per unit surface on the glass surrounding the coiled electrode section, it is not frequently used in high intensity discharge lamp products. One reason for this is a dose loss in the micro channels surrounding the coil component in the glass wall. During lamp operation, the dose constituents slowly migrate outwards from the discharge chamber and fill the micro channels around the coil on the electrode in the seal. The result of this dose migration is a gradual change in lamp parameters. This is because the amount of the dose in the arc chamber and its temperature (the ‘cold spot temperature’) are important factors which determine the electrical and optical parameters of the lamp, especially the color performance and the luminous flux of metal halide lamps. Such a gradual—and often very rapid—change in lamp performance caused by the significant dose loss in the micro channels is unacceptable.
The other result of dose loss in micro channels surrounding the coil on the electrode in the seal is the build-up of a dose reservoir in the micro channels. Since the thermal expansion coefficient of e.g. the metal halide dose component can be greater by orders of magnitude than that of the quartz glass surrounding the channels, cracks may be generated by the mechanical stresses from this thermal expansion mismatch between the quartz glass and the metal halide dose components in the reservoir. Finally, the lamp may become leaking and inoperative, or can even be ruptured.
Thus, there is a particular need to provide a high intensity discharge lamp with electrodes limiting the temperature of the electrode foot-points by enhanced heat dissipation (mainly by radiation and additionally by convection/conduction through the surrounding discharge gas and vapour in the discharge vessel) along the electrode shaft within the discharge vessel. There is also a need for a simpler foot-point temperature limiting structure than that with the embedded coil. There is a further need to provide such a lamp with an electrode structure that has no additional elements close to its tip portion pointing towards the central region of the discharge vessel.